In the Cebidae there is an extra premolar in each jaw bringing the number up to 36. In the Hapalidae, as in the Cebidae, there is a third premolar, but the molars are reduced to 2/2. Man is the only Primate that has the teeth arranged in a continuous series. In all the others there is a gap or diastema of larger or smaller size between the incisors and canines. In all except man also the canines are enlarged, especially in the males. The Exoskeletal structures of mammals may be summarised in the following table: I. _Epidermal exoskeletal structures._ 1. Hairs (_a_) ordinary hair, (_b_) vibrissae and bristles, (_c_) spines of hedgehog, porcupine, _Echidna_, _Centetes_, _Acanthomys_. 2. Scales { of Manidae, { on tails of rats, beavers, &c. 3. Horns of Rhinoceros. 4. Horns of Bovine Ruminants. 5. Nails, claws, hoofs. 6. Spurs of male _Ornithorhynchus_ and _Echidna_. 7. Horny beak and teeth of _Ornithorhynchus_. 8. Horny pads on jaws of Sirenians and Ruminants. 9. Baleen of whales. 10. Enamel of teeth. II. _Dermal exoskeletal structures._ 1. Dentine and cement of teeth. 2. Bony scutes of Armadillos. ENDOSKELETON. VERTEBRAL COLUMN. CERVICAL VERTEBRAE. The cervical vertebrae of all mammals have certain characters in common. However long the neck may be, the number of cervical vertebrae, with very few exceptions, is seven. Movable ribs are generally absent, and if present are small and do not reach the sternum. The transverse processes are generally wide but not long, and are perforated near the base by the vertebrarterial canals, through which the vertebral arteries pass; they generally bear downwardly directed inferior lamellae which are sometimes as in the seventh human cervical seen to ossify from centres distinct from those forming the rest of the transverse process, and are really of the nature of ribs. The atlas and axis always differ much from the other vertebrae.